Thursday, October 29, 2009

Baryon Acoustic Oscillations

Baryon Acoustic Oscillations
Baryon Acoustic Oscillations(BAO) are frozen relics left over from the pre-decoupling universe. They are the standard rulers of choice for 21st century cosmology, providing distance estimates that are, for the first time, firmly rooted in well-understood, linear physics. This review synthesises current understanding regarding all aspects of BAO cosmology, from the theoretical and statistical to the observational, and includes a map of the future landscape of BAO surveys, both spectroscopic and photometric.

Monday, October 26, 2009

Monty Hall and John Von Neumann walk into a bar ...

I recently read The Monty Hall Problem: The Remarkable Story of Math's Most Contentious Brain Teaser .
Here's my version of the Monty Hall Problem.
The Basic Situation is as follows. There are two individuals: Monty Hall and Alice. There is a car; there are three closed curtains; and the car is behind one of the curtains, which hides it completely. There is nothing behind the other two curtains. The action proceeds as follows:
1. Monty hides the car behind one of the curtains (H); Alice has no idea which one.
2. Alice chooses a curtain (C), but it is left closed. Alice doesn't know yet whether she chose the car or not.
3. Monty opens one of the curtains (S) showing Alice what's behind it.
3a. Monty is not permitted to open curtain Alice's curtain C; S cannot equal C.
4. Alice finally chooses another curtain (F) which can be different than C or the same. Alice gets what's behind the curtain she finally chose. If F=H, Alice wins the car, otherwise Alice gets nothing.

We can nail this down so that it is an exercise in pure logic and probability for Alice by further specifying what Monty does at steps 1 and 3. Here are the Additional Stipulations.
1a. Monty chooses where to hide the car (H) by randomly picking one of the three curtains: each of the three curtains is equally likely.
3b. Monty will only show Alice an empty curtain in step 3. Monty never opens the curtain with the car. S cannot equal H.

Given these Additional Stipulations. the consequences of the Alice's choice in step 4 are completely unambiguous - however the results surprise many people. Alice's two main strategies are Stay (F=C) and Switch (F≠C≠S) Many people guess that Stay and Switch are equivalent and that Alice wins 1/2 the time either way. Surprisingly Stay only wins 1/3 of the time while Switch wins 2/3's of the time.

Suppose Alice elects to follow the following strategy: always choose curtain #1 in step 2 and always Stay with curtain #1 in step 4.
In step 1. Monty hides the car behind curtain #1 1/3 of the time.
In step 2. Alice always chooses curtain #1.
In step 3. Monty will open either curtain #2 or curtain #3. Note that this does not change the actual location of the car, it's still behind curtain #1.
In step 4. Alice will always choose curtain #1 again. Alice wins the car.

In step 1. Monty hides the car behind curtain #2 or curtain #3 2/3's of the time.
In step 2. Alice always chooses curtain #1, which is empty.
In step 3. Monty will open either curtain #2 or curtain #3. Note that this does not change the location of the car, curtain #1 is still empty.
In step 4. Alice will always choose curtain #1 again, which is empty. Alice gets nothing.

So we see that by following the strategy of always chosing curtain #1 both times, Alice only wins the car 1/3 of the time.

Suppose on the other hand Alice uses another strategy: in step 2. she always chooses curtain #1; in step 4. shes always Switches to the only other curtain which is still closed.

In step 1. Monty hides the car behind curtain #1 1/3 of the time.
In step 2. Alice always chooses curtain #1.
In step 3. Monty will always open curtain #2 or curtain #3. Note that this does not change the actual location of the car, it's still behind curtain #1.
In step 4. because Alice always switches she will choose either curtain #2 or curtain #3. But the car is still behind curtain #1 and Alice gets nothing.

In step 1. Monty hides the car behind curtain #2 1/3 of the time.
In step 2. Alice always chooses curtain #1, which is empty.
In step 3. Monty must open curtain #3 - because it is the only door which is empty and is not Alice's. That of course does not change the location of the car - which is still behind curtain #2.
In step 4. Alice always switches to curtain #2 - because it is still closed. Alice wins.

In step 1. Monty hides the car behind curtain #3 1/3 of the time.
In step 2. Alice always chooses curtain #1, which is empty.
In step 3. Monty must open curtain #2 - because it is the only door which is empty and is not Alice's. That of course does not change the location of the car - which is still behind curtain #3.
In step 4. Alice always switches to curtain #3 - because it is still closed. Alice wins.

So Alice always wins if Monty hid the car behind curtain #2 or curtain #3 and Alice always loses if Monty hid the car behind curtain #1. Perhaps suprisingly Alice wins 2/3's the time when she always switches.


There's another formulation of the problem which only uses the facts in the Basic Situation. The Additional Stipulations are not included. Surprisingly Alice can guarentee the same favorable outcome in the Basic Situation that was achievable with the Additional Stipulations! Monty is permitted to choose where to hide the car (H) in step 1. any way he likes. In step 3. he is permitted to choose which curtain to open (S) by any method, as long as he doesn't open curtain C (still forbidden by 3a). It doesn't matter how Monty makes his choices (as long as he obeys 3a), Alice can still win the car surprisingly often.

Here's a paper (in pdf) which explains the Game Theory approach to the Monty Hall Problem: Probabilistic and Game Theoretic solutions to The Three Doors Problem

Friday, October 23, 2009

Quasars

Pending problems in QSOs
Quasars (Quasi Stellar Objects, abbreviated as QSOs) are still nowadays, close to half a century after their discovery, objects which are not completely understood. In this brief review a description of the pending problems, inconsistencies and caveats in the QSO's research is presented. The standard paradigm model based on the existence of very massive black holes that are responsible for the QSO's huge luminosities, resulting from to their cosmological redshifts, leaves many facts without explanation. There are several observations which lack a clear explanation, for instance: the absence of bright QSOs at low redshifts, a mysterious evolution not properly understood; the inconsistencies of the absorption lines, such as the different structure of the clouds along the QSO's line of sight and their tangential directions; the correlation of redshifts between QSOs and galaxies; and many others.

Nonrelativistic Quantum Gravity

Aspects of nonrelativistic quantum gravity Physics preprint.
A nonrelativistic approach to quantum gravity is studied. At least for weak gravitational fields it should be a valid approximation. Such an approach can be used to point out problems and prospects inherent in a more exact theory of quantum gravity, yet to be discovered. Nonrelativistic quantum gravity, e.g., shows promise for prohibiting black holes altogether (which would eliminate singularities and also solve the black hole information paradox), gives gravitational radiation even in the spherically symmetric case, and supports non-locality (quantum entanglement). Its predictions should also be testable at length scales well above the "Planck scale", by high-precision experiments feasible with existing technology.

Tuesday, October 20, 2009

Monday, October 19, 2009

Quantum Effects in Biology

Some quantum weirdness in physiology
Quantum mechanics seems alien to physiology. Alarm bells go off in our heads when we hear even people of such genius as Sir Roger Penrose (1) invoke the weird coherence of quantum mechanical wave functions to explain biological function. Of course, it is only some of the “weirder” parts of quantum mechanics that bother us. Structural biochemistry is founded on the rigid geometrical relationships involved in chemical bonding that arise from quantum mechanics; the α-helix could only have been discovered by Pauling by acknowledging the power of quantum mechanical resonance to flatten the peptide bonding unit (2). Nevertheless, most modern biomolecular scientists view quantum mechanics much as deists view their God; it merely sets the stage for action and then classically understandable, largely deterministic, pictures take over. In this issue of PNAS Ishizaki and Fleming (3), by combining experimental and theoretical investigations, demonstrate that quantum coherence effects play a big role in light energy transport in photosynthetic green sulfur bacteria under physiological conditions. Quantum coherence allows a nonclassical simultaneous exploration of many paths of energy flow through the many chromophores of a light-harvesting complex, thereby significantly increasing the efficiency of the energy capture process, presumably helping the bacteria to survive in low light.

Cosmic Reionization

Reionization and Cosmology with 21 cm Fluctuations
Three major stages in the evolution of our universe are written in the phases of hydrogen. After nucleosynthesis the universe was an ionized plasma of hydrogen and helium. As expansion cooled the universe, hydrogen went through a phase transition, rapidly becoming neutral and releasing the Cosmic Microwave Background light at a redshift of ~1089. High energy photons produced by the firrst stars and quasars later reionized the hydrogen in the inter galactic medium (IGM), forcing the universe back through a second extended and patchy phase transition referred to as the epoch of reionization (EoR).

Saturday, October 17, 2009

Morning Glory

Rhyolitic volcanoes

Rhyolitic volcanoes: the ones to watch in Nature.
The Chaitén volcano in Chile erupted unexpectedly and explosively on 1 May 2008, and it is still erupting. The eruption has displaced over 5,000 people, and resulted in millions of dollars of lost revenue in Chile. It has also provided geophysicists the rare opportunity of directly observing a rhyolite magma fuelled eruption — the cause of some of Earth's largest explosive volcanic eruptions. Jonathan Castro and Donald Dingwell present petrological and experimental evidence to show that the hydrous rhyolite magma at Chaitén ascended very rapidly, with velocities of the order of a metre per second. Such rapid ascent, contrasting markedly with the behaviour of most silicic magmas, implies a transit time from storage depths greater than 5 km to the near surface of only 4 hours, leaving little warning time for such eruptions. This work suggests that rhyolitic volcanoes that have been active during the Holocene — the past 10,000 years or so — should be closely monitored, especially those near major centres of population.

Clovis Era Impact Fails to be Confirmed

Impact theory under fire once more in Nature.
Independent studies are casting more doubt on a controversial theory that a comet exploded over North America nearly 13,000 years ago, wiping out the Clovis people and many of the continent's large animals.

Thursday, October 15, 2009

Fractional quantum Hall Effect Observed in Graphene

Fractional quantum Hall effect and insulating phase of Dirac electrons in graphene
In graphene, which is an atomic layer of crystalline carbon, two of the distinguishing properties of the material are the charge carriers two-dimensional and relativistic character. The first experimental evidence of the two-dimensional nature of graphene came from the observation of a sequence of plateaus in measurements of its transport properties in the presence of an applied magnetic field. These are signatures of the so-called integer quantum Hall effect. However, as a consequence of the relativistic character of the charge carriers, the integer quantum Hall effect observed in graphene is qualitatively different from its semiconductor analogue. As a third distinguishing feature of graphene, it has been conjectured that interactions and correlations should be important in this material, but surprisingly, evidence of collective behaviour in graphene is lacking. In particular, the quintessential collective quantum behaviour in two dimensions, the fractional quantum Hall effect (FQHE), has so far resisted observation in graphene despite intense efforts and theoretical predictions of its existence. Here we report the observation of the FQHE in graphene. Our observations are made possible by using suspended graphene devices probed by two-terminal charge transport measurements. This allows us to isolate the sample from substrate-induced perturbations that usually obscure the effects of interactions in this system and to avoid effects of finite geometry. At low carrier density, we find a field-induced transition to an insulator that competes with the FQHE, allowing its observation only in the highest quality samples. We believe that these results will open the door to the physics of FQHE and other collective behaviour in graphene.

Physics from Pure Geometry

Matter from Space
General Relativity offers the possibility to model attributes of matter, like mass, momentum, angular momentum, spin, chirality etc. from pure space, endowed only with a single field that represents its Riemannian geometry. I review this picture of `Geometrodynamics' and comment on various developments after Einstein.

Monday, October 12, 2009

Gogol Bordello

Here's a goofy video by the band >Gogol Bordello - Supertheory of Supereverything - liberally sprinkled with physics jargon.

Saturday, October 10, 2009

Unseen expressions trigger emotional reactions

Unseen facial and bodily expressions trigger fast emotional reactions
Facial reactions were recorded using electromyography, and arousal responses were measured with pupil dilatation. Passive exposure to unseen expressions evoked faster facial reactions and higher arousal compared with seen stimuli, therefore indicating that emotional contagion occurs also when the triggering stimulus cannot be consciously perceived because of cortical blindness. Furthermore, stimuli that are very different in their visual characteristics, such as facial and bodily gestures, induced highly similar expressive responses

Learning about Odors Requires Generating New Neurons

Olfactory perceptual learning requires adult neurogenesis in PNAS
Perceptual learning is required for olfactory function to adapt appropriately to changing odor environments. We here show that newborn neurons in the olfactory bulb are not only involved in, but necessary for, olfactory perceptual learning. First, the discrimination of perceptually similar odorants improves in mice after repeated exposure to the odorants. Second, this improved discrimination is accompanied by an elevated survival rate of newborn inhibitory neurons, preferentially involved in processing of the learned odor, within the olfactory bulb. Finally, blocking neurogenesis before and during the odorant exposure period prevents this learned improvement in discrimination. Olfactory perceptual learning is thus mediated by the reinforcement of functional inhibition in the olfactory bulb by adult neurogenesis.

How can we measure Galaxy distances?

Cosmology: Dark is the new black in Nature.
Rival experimental methods to determine the Universe's expansion are contending to become the fashionable face of cosmology. Fresh theoretical calculations make one of them the hot tip for next season.

It's difficult to determine distances in astronomy. Distances to galaxies have been measured using three techniques: Type 1a supernova; gravitational lensing; baryon acoustic oscillations - sound waves from the big bang.

Color Blindness Cure in Monkeys

Vision: Gene therapy in colour
Replacing a missing gene in adult colour-blind monkeys restores normal colour vision. How the new photoreceptor cells produced by this therapy lead to colour vision is a fascinating question.

The Monty Hall Problem: A New Book

Two Doors and a Goat is a review in Science Magazine of The Monty Hall Problem: The Remarkable Story of Math's Most Contentious Brain Teaser by Jason Rosenhouse.

Thursday, October 08, 2009

Measurement-based quantum computation

Measurement-based quantum computation
Quantum computation offers a promising new kind of information processing, where the non-classical features of quantum mechanics can be harnessed and exploited. A number of models of quantum computation exist, including the now well-studied quantum circuit model. Although these models have been shown to be formally equivalent, their underlying elementary concepts and the requirements for their practical realization can differ significantly. The new paradigm of measurement-based quantum computation, where the processing of quantum information takes place by rounds of simple measurements on qubits prepared in a highly entangled state, is particularly exciting in this regard. In this article we discuss a number of recent developments in measurement-based quantum computation in both fundamental and practical issues, in particular regarding the power of quantum computation, the protection against noise (fault tolerance) and steps toward experimental realization. Moreover, we highlight a number of surprising connections between this field and other branches of physics and mathematics.

The Flyby Anomaly

Earth Flyby Anomalies
In a reference frame fixed to the solar system's center of mass, a satellite's energy will change as it is deflected by a planet. But a number of satellites flying by Earth have also experienced energy changes in the Earth-centered frame -- and that's a mystery.

Tuesday, October 06, 2009

Phoenix Universe?

Title: The Return of the Phoenix Universe: physic preprint.
Georges Lemaitre introduced the term "phoenix universe" to describe an oscillatory cosmology with alternating periods of gravitational collapse and expansion. This model is ruled out observationally because it requires a supercritical mass density and cannot accommodate dark energy. However, a new cyclic theory of the universe has been proposed that evades these problems. In a recent elaboration of this picture, almost the entire universe observed today is fated to become entrapped inside black holes, but a tiny region will emerge from these ashes like a phoenix to form an even larger smooth, flat universe filled with galaxies, stars, planets, and, presumably, life. Survival depends crucially on dark energy and suggests a reason why its density is small and positive today.

Monday, October 05, 2009

The Deep Ocean was Oxygenated Late

Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age in PNAS.
Molecular oxygen (O2) began to accumulate in the atmosphere and surface ocean ca. 2,400 million years ago (Ma), but the persistent oxygenation of water masses throughout the oceans developed much later, perhaps beginning as recently as 580–550 Ma. For much of the intervening interval, moderately oxic surface waters lay above an oxygen minimum zone (OMZ) that tended toward euxinia (anoxic and sulfidic).

Economic Downturns are Good for Your Health?

Life and death during the Great Depression in PNAS.
Recent events highlight the importance of examining the impact of economic downturns on population health. The Great Depression of the 1930s was the most important economic downturn in the U.S. in the twentieth century. We used historical life expectancy and mortality data to examine associations of economic growth with population health for the period 1920–1940. We conducted descriptive analyses of trends and examined associations between annual changes in health indicators and annual changes in economic activity using correlations and regression models. Population health did not decline and indeed generally improved during the 4 years of the Great Depression, 1930–1933, with mortality decreasing for almost all ages, and life expectancy increasing by several years in males, females, whites, and nonwhites. For most age groups, mortality tended to peak during years of strong economic expansion (such as 1923, 1926, 1929, and 1936–1937). In contrast, the recessions of 1921, 1930–1933, and 1938 coincided with declines in mortality and gains in life expectancy. The only exception was suicide mortality which increased during the Great Depression, but accounted for less than 2% of deaths. Correlation and regression analyses confirmed a significant negative effect of economic expansions on health gains. The evolution of population health during the years 1920–1940 confirms the counterintuitive hypothesis that, as in other historical periods and market economies, population health tends to evolve better during recessions than in expansions.

Cosmic Rays - mysteries of the sources

IceCube: The Rationale for Kilometer-Scale Neutrino Detectors
At a time when IceCube is nearing completion, we revisit the rationale for constructing kilometer-scale neutrino detectors. We focus on the prospect that such observatories reveal the still-enigmatic sources of cosmic rays. While only a "smoking gun" is missing for the case that the Galactic component of the cosmic-ray spectrum originates in supernova remnants, the origin of the extragalactic component remains a mystery. We speculate on neutrino emission from gamma-ray bursts and active galaxies.

Supernova Cosmology

Foundations of Supernova Cosmology
This is a brief sketch of the use of supernovae to measure cosmological parameters. It traces the early work, the events surrounding the discovery and verification of cosmic acceleration using SN Ia, and the efforts today to make sound inferences about the nature of dark energy. The prospects for minimizing systematics by using near-infrared observations in the supernova restframe are emphasized. This could be an important point in the design of a JDEM that employs supernovae to measure the history of cosmic expansion.

Tycho's Nova

The Periodic Table and Group Theory

From the Mendeleev periodic table to particle physics and back to the periodic table
We briefly describe in this paper the passage from Mendeleev's chemistry (1869) to atomic physics (in the 1900's), nuclear physics (in the 1932's) and particle physics (from 1953 to 2006). We show how the consideration of symmetries, largely used in physics since the end of the 1920's, gave rise to a new format of the periodic table in the 1970's. More specifically, this paper is concerned with the application of the group SO(4,2)xSU(2) to the periodic table of chemical elements. It is shown how the Madelung rule of the atomic shell model can be used for setting up a periodic table that can be further rationalized via the group SO(4,2)xSU(2) and some of its subgroups. Qualitative results are obtained from this nonstandard table.

Friday, October 02, 2009

Anxiety

Understanding the Anxious Mind an article in the New York Times profiling the research of Harvard psychologist Jerome Kagan.

Thursday, October 01, 2009

Value at Risk

Simon Johnson discusses the Value at Risk (VAR) financial analysis technique and other issues in financial modelling in The Economics of Models

A Harvard Critique

The 'Veritas' About Harvard
Harvard has consistently admitted around 1600 undergraduates per year since 1990. The article argues that some of Harvard's wealth might be better spent by increasing undergraduate enrollment.